1. The Field of the Invention
The present invention relates to an apparatus and method for measuring the concentration of various components in a fluid, and more particularly, to a field effect transistor which is chemically sensitive.
2. The Prior Art
There have been many different types of field effect transistors (commonly referred to as "FET") developed for various applications, a few of which are chemically sensitive. Those FETs which are chemically sensitive are often referred to as "CHEMFETs." One type of CHEMFET is an ion-sensitive transistor which was developed for measuring such chemical properties as ion activity and ion concentration in an ion-containing liquid. See, for example, U.S. Pat. No. 4,020,830 entitled "Selective Chemical Sensitive FET Transducers" which issued on May 3, 1977 to Johnson et al., which patent is incorporated herein by reference.
Other electrically sensitive devices have been developed for similar purposes, e.g., a metal oxide semiconductor field effect transistor or "MOSFET" device. See, for example, Piet Bergveld, "Development, Operation, And Application of the Ion-Sensitive Field-Effect Transistor as a Tool for Electrophysiology," IEEE Transactions of Biomedical Engineering at 342-51 (September 1972). Although ion-sensitive CHEMFET devices such as those disclosed in U.S. Pat. No. 4,020,830 solve many of the problems experienced by the MOSFET devices disclosed in the above-cited Bergveld article, both of these ion-sensitive CHEMFET and MOSFET devices are limited to the analysis of liquids which are capable of conducting an electrical current.
The need for an apparatus which is capable of measuring the concentration of various components in a nonconducting liquid is readily apparent. For example, it may be desirable to analyze certain liquid petroleum products to determine the amount of impurities contained therein. The prior art ion-sensitive transistors would not be useful for this type of analysis, because a conducting liquid is generally required for the operation of an ion-sensitive transistor and petroleum products are generally nonconductive.
Thus, it will be readily appreciated that what is needed in the art is an apparatus for measuring the concentration of components in a nonconducting liquid. Such an apparatus is embodied in the present invention and serves to complement the prior art by doing what the prior art could not, namely, measure the concentration of components in a nonconducting liquid.
Another limitation of the FET devices known in the art is their limited application in measuring the concentration of impurities and other components in gaseous fluids. Existing devices are generally very specific to the types of gaseous components which can be detected; hence, their versatility is severely limited.
For example, one device has been developed for measuring the concentration of hydrogen in a gaseous sample. (See I. Lundstrom, "A Hydrogen-Sensitive MOS Field-Effect Transistor," 26 Applied Physics Letters 55-57 (Jan. 15, 1975).) This device is a MOSFET which incorporates a palladium layer to adsorb and dissolve the hydrogen in the gaseous sample. After adsorption and dissolution of the molecular hydrogen gas into the palladium film, the hydrogen molecules dissociate into atomic hydrogen and the dipole moments of the hydrogen atoms cause a change in the work function of the palladium metal. Thus, by measuring the change in the electric potential of such a device, the concentration of hydrogen gas in the sample can be determined.
It will be readily apparent that the above-described hydrogen measuring FET device is limited to detecting only those gaseous components which can penetrate the palladium layer, namely hydrogen. Such a device is obviously not suitable for more general applications of measuring the concentration of other gaseous components.
Another type of device has also been developed which measures the concentration of gaseous components based on changes in the work function of an adsorbing conductor. This device measures the magnitude of the change in work function as the so-called volta potential by a vibrating capacitor. See, for example, G. Phillips, "An Electronic Method of Detecting Impurities in the Air," 28 J. Sci. Inst. 342-47 (1951).
Still another type of device which is used for measuring the concentration of various reducible gaseous components is a conductivity measuring apparatus. One such device is disclosed in U.S. Pat. No. 3,719,564. The device disclosed in that patent includes a solid-state electrochemical cell having a pair of electrodes and a rare earth fluoride electrolyte sandwiched therebetween. The concentration of certain reducible gases is measured by exposing the cell to the gaseous sample and recording the cell current which is a function of the concentration of the reducible gases.
Devices such as that described in U.S. Pat. No. 3,719,564 are not only limited to the concentration measurement of reducible gases, but also to those reducible gases which are capable of penetrating the electrolyte material. Thus, any gases which are not capable of sufficiently penetrating the electrolyte go undetected. Moreover, if several reducible gases are present in the gaseous sample analyzed, such a device is incapable of selectively measuring the concentration of each individual gaseous component.
It will thus be appreciated that it would also be an advancement in the art to provide an apparatus which does not require penetration of the gaseous sample into a material within the apparatus so as to render the apparatus capable of measuring the concentration of a variety of different gaseous components, thus providing for general application. It would be a further advancement in the art to provide such an apparatus which could be adapted to selectively measure the concentration of one or more individual gaseous components in a gaseous sample. Such an apparatus and method for measuring the concentration of components in both gaseous and liquid fluids is disclosed and claimed herein.